To estimate \(E(B-V)\)  via the Chołoniewski method one needs to know the spectral shape of the variable component of an unreddened AGN. The optical-to-UV emission of a thermal AGNSee [1]} for discussion of the division of AGNs into thermal (high accretion rate) and non-thermal (low accretion rate) AGNs. is dominated by thermal emission from the accretion disc. The outer parts of the disc are externally illuminated by emission just above the disc plane closer to the centre of the disc. This produces a spectrum with a power-law dependence of the flux per unit frequency on the frequency: \(F_{\nu } \propto \nu ^{+1/3}\)  [2]}Coincidentally and confusingly, this is also the better-known theoretical spectrum produced by the outer parts of the standard internally-heated accretion disc of [3]}.. [4]} show that the distribution of spectral indices of the variable component of the optical-to-near-UV spectra in over 4000 SDSS AGNs is consistent with the unreddened continuum having the expected \(^{+1/3}\)  spectrum after allowance for BLR emission. The intrinsic ratio of the unreddened fluxes per unit frequency between two wavelengths of the disc component alone is thus expected to be
\(~~~~~~~~~~~~~~~~~~~~\left[ \frac{F(_1)}{F(_2)}\right]_{\mathrm {int}} = \left[\frac{_2}{_1}\right]^{+1/3} .\) 
